1. Field of the Invention
This invention relates to a process for producing alkoxysilane cluster compounds.
2. Description of the Prior Art
U.S. Pat. No. 3,965,136, which issued to the present inventor on June 22, 1976, disclosed the preparation of alkoxysilane cluster compounds of the formula: EQU RSi[OSi(OR').sub.3 ].sub.3 (I)
wherein R is hydrogen, an alkyl, alkenyl, aryl, or aralkyl; and each R' is independently selected from the same groups as R with the proviso that at least a majority of the R' radicals are sterically hindered alkyl groups having at least 3 carbon atoms. These alkoxysilane compounds of formula (I) have been disclosed to be good functional fluids because of their unique combination of properties. Potential applications include high performance hydraulics, heat transfer agents, greases, dielectric coolants, and as a formulating aid in combination with other fluids.
In particular, two different processes for preparing these alkoxysilane cluster compounds are disclosed in U.S. Pat. No. 3,965,136. The first method of preparation involves the reaction of a trihalosilane with a trialkoxysilanol in the presence of an acid acceptor like pyridine. The second disclosed method of preparation involves the reaction of halosilane with an alkoxysilanol cluster compound in the presence of an acid acceptor.
An optimization of the above-noted first method of preparation is disclosed in U.S. Pat. No. 4,077,993, which also issued to the present inventor on Mar. 7, 1978. This improved method of preparation involves reacting a trihalosilane with a trialkoxysilanol in the presence of critical amounts of acceptor base and solvent reaction medium while maintaining the reaction temperature in a select range.
It has been found that these two methods for preparing alkoxysilane cluster compounds of formula (I) are somewhat hindered by the fact that they require an acid acceptor. Suitable acid acceptors such as pyridine are relatively expensive and must be recovered from the reaction mixture in order to reduce the cost of the synthesis. However, the recovery steps themselves are expensive to run on a large scale. Also, the yield of the alkoxysilane cluster product may be reduced because the occlusion of some of the product to the acid acceptor salt, even after washing steps. Furthermore, organic acid acceptors such as pyridine and the like have unpleasant physical characteristics and their use may present a health hazard in some circumstances.
Furthermore, it has been found that the use of certain trihalosilane intermediates does not result in desirably high yields of alkoxysilane cluster compounds for commerical use. The exact reasons for this is not known, but most likely involves steric hindrance of the reaction transition complexes of reactants and acid acceptor.
For the above and other reasons, a method for preparing alkoxysilane cluster compounds is needed which is relatively inexpensive and which does not require the use of pyridine or any other type of acid acceptor compound. The present invention, as described in detail below, presents a solution to this need.
Separately, polyalkoxy di-, tri-, and tetrasiloxanes have been prepared by reacting the corresponding alkoxysilanol with an appropriate alkoxysilamine. See U.S. Pat. No. 2,758,127, which issued to Goldschmidt et al on Aug. 7, 1956. However, this disclosed reaction was directed toward making linear-structured siloxanes, and not branched-structured compounds having sterically hindered alkoxy groups as is the present case.